燈塔照明

Light Up: rules and solving techniques

Place non-conflicting bulbs to light every white cell and satisfy the clues. This guide reflects the controls and win conditions in the current browser build. It is reviewed by the PocketJoy game-testing team and maintained by Long-Term New Media Limited.

Puzzle rules

An Light Up board, also known as Light Up, is made of white and black cells. You place bulbs on white cells so that every white cell on the board ends up lit. A bulb casts light along its own row and column in all four directions until a black cell blocks it, and the cell holding the bulb is of course lit as well. Two restrictions apply. First, no two bulbs may see each other — if two cells share a row or column with no black cell between them, they cannot both hold bulbs. Second, some black cells carry a number from 0 to 4 stating exactly how many of their four orthogonally adjacent cells hold bulbs, while black cells without a number are unconstrained. The puzzle is complete when every white cell is lit, no bulbs see one another, and every numbered black cell is satisfied. Three difficulties are available, and the check button reports whether cells remain unlit, in conflict, or in violation of a number.

Reasoning and solving techniques

Working from the numbered black cells is the most reliable opening, starting with the extremes. A cell marked 0 can have no bulb on any of its four neighbours, so mark those four as forbidden — free, certain information. A cell marked 4 has bulbs on all four neighbours, which you can place immediately. Then look for constrained high numbers: a 3 against the board edge has only three adjacent white cells, so all three must be bulbs, and a 2 with only two adjacent white cells is equally forced. With those placed, turn to the lighting constraint and scan for white cells that can only be lit one way. The typical case is a dead-end cell enclosed on three sides by black cells: it can only be lit by itself or from the single open direction, and if that direction is also blocked, the cell itself must hold a bulb. Another efficient technique is elimination in reverse: if placing a bulb on a given cell would make it see an existing bulb, that cell can never hold one, and marking it forbidden shifts the responsibility for lighting it onto other cells, which often cascades into several further deductions.

Easy mistakes to make

The most common mistake is concentrating on lighting cells and forgetting that bulbs must not see one another. Many players place bulbs until the board glows, then discover on checking that two bulbs face each other along a row with no black cell between them, forcing a wholesale rebuild. Sweeping the four directions after each placement is a necessary habit. The second mistake is forgetting that unnumbered black cells carry no constraint; some players treat every black cell as demanding adjacent bulbs, when only numbered ones do and the rest are simply walls. The third is placing bulbs without ever marking forbidden cells. Knowing where a bulb cannot go is as valuable as knowing where one must, particularly when working out which cell is responsible for lighting another, and without those marks you will re-derive the same region repeatedly. The fourth is guessing: the solution is unique, a wrong guess surfaces its contradiction far away, and backtracking is expensive.

Who will enjoy it

This puzzle type was introduced by the Japanese puzzle publisher Nikoli, which publishes it under the name Akari, and it belongs to the same family as sudoku and the loop puzzles, but it feels quite different to solve. Where sudoku handles numeric constraints, Light Up deals in sightlines and coverage, which is closer to spatial intuition than to arithmetic. That makes it unusually friendly to people who enjoy logic but dislike calculation — nothing in the game requires adding anything, only counting a few neighbours and judging whether a line of sight is blocked. It suits players who enjoy the moment when everything falls into place: Light Up boards frequently resolve in cascades, with one key bulb instantly fixing a whole chain of cells, and that cascade is the game's real pleasure. There is no luck, each puzzle has a unique solution, and every position is deducible without guessing. Easy puzzles take a few minutes while hard ones demand a more systematic order. Nothing counts down, and the timer is only for comparing with yourself.

Helper features

Three difficulties are available with a fastest time stored for each. The sidebar's bulb counter shows how many you have placed, which becomes useful later in a puzzle: once you know how many the solution requires, the placed count tells you how many remain. The timer panel shows your elapsed time on the current puzzle beside your best for that difficulty. The check button validates three conditions at once — whether everything is lit, whether any bulbs conflict, and whether the numbered cells are satisfied — and when it fails it states specifically that cells remain unlit, in conflict or mismatched, so you know which category of problem to look for rather than receiving a vague failure. A hint button settles one step. Lit and unlit white cells are visually distinct, so you can see directly which regions remain dark without tracing every bulb's sightlines yourself. Completion shows your total time and offers a fresh puzzle.

Controls and saved progress

There is a single interaction: tap a white cell to place a bulb, and tap the same cell again to remove it, with no mode to switch. Black cells cannot be tapped and are ignored automatically, so mis-taps are not a concern. Mouse and touch follow identical logic, and cell sizes are tuned so a whole board can be worked one-handed on a phone. Since a misplaced bulb costs nothing more than another tap to remove, you can experiment with layouts freely. Fastest times per difficulty are stored in the browser's local storage on your own device, never uploaded to any server and requiring no account; records do not travel to another device, and clearing browser data removes them. The language button switches between Traditional Chinese and English instantly, updating every prompt and check result. Once the page has loaded, the game itself plays offline.

Solving FAQ

Reviewed by the PocketJoy game-testing team · Operated by Long-Term New Media Limited · hklongtermad@gmail.com